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Facile synthesis of ZnS nanoparticles decorated on defective CNTs with excellent performances for lithium-ion batteries anode material
Journal Pre-proof Facile synthesis of ZnS nanoparticles decorated on defective CNTs with excellent performances for lithium-ion batteries anode material Wenlong Zhang, Zhongyuan Huang, Haihui Zhou, Songlin Li, Chuqing Wang, Huanxin Li, Zhanheng Yan, Fei Wang, Yafei Kuang PII:
S0925-8388(19)33879-4
DOI:
https://doi.org/10.1016/j.jallcom.2019.152633
Reference:
JALCOM 152633
To appear in:
Journal of Alloys and Compounds
Received Date: 20 July 2019 Revised Date:
30 September 2019
Accepted Date: 9 October 2019
Please cite this article as: W. Zhang, Z. Huang, H. Zhou, S. Li, C. Wang, H. Li, Z. Yan, F. Wang, Y. Kuang, Facile synthesis of ZnS nanoparticles decorated on defective CNTs with excellent performances for lithium-ion batteries anode material, Journal of Alloys and Compounds (2019), doi: https:// doi.org/10.1016/j.jallcom.2019.152633. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Facile synthesis of ZnS nanoparticles decorated on defective CNTs with excellent performances for lithium-ion batteries anode material Wenlong Zhanga, Zhongyuan Huang*a, Haihui Zhou*a, Songlin Li*b, Chuqing Wanga, Huanxin Lia, Zhanheng Yana, Fei Wanga, Yafei Kuang*a
a
State Key Laboratory for Chemo/Biosensing and Chemometrics, College of
Chemistry and Chemical Engineering, Hunan University, Changsha, Hunan, 410082, China b
State Key Laboratory of Powder Metallurgy, Central South University, Changsha,
Hunan, 410083, China *
Corresponding authors: ([email protected]; [email protected]; [email protected])
[email protected];
Abstract
Zinc sulfide (ZnS) is expected to be a potential replacement of commercial graphite as the anode material for lithium-ion batteries (LIBs) due to its non-toxicity, high theoretical capacity and low cost. Nevertheless, there are still several problems for the application of ZnS during lithium-ion storage process, such as undergoing intercalation/extraction reaction, conversion reaction (ZnS to Zn and LiS2) and alloying reaction (LixZn), which make ZnS exhibit more pronounced polarization and lower reversible capacity at high current density. In order to overcome these problems, in this work we designed a composite (ZnS-CNTs) of etched multi-walled carbon nanotubes (CNTs) and ZnS nanoparticles used as the anode material of LIBs. CNTs were etched in air to obtain rough surfaces with many defects, which could provide many active sites for the growth of ZnS nanoparticles. The ZnS-CNTs composite exhibits high reversible capacity of 451.3 mAh/g after 1200 cycles at a high current density of 5 A/g, and superior rate capability (377.8 mAh/g at 8 A/g). Interestingly, the ZnS-CNTs electrode exhibits the typical specific capacity recovery phenomenon, resulting in an excellent stability during the long-term cycling. SEM images of the ZnS-CNTs electrode materials after different cycles show that the ZnS nanoparticles and CNTs gradually merged together with the increase of cycles, which
not only effectively alleviates the structural damage caused by volume change of ZnS in the charge and discharge cycling, but also greatly increases the specific capacity due to the increase of its electrochemical reaction area.
Keywords: Li ion battery
Anode material
ZnS
Defective CNTs
1. Introduction
Over the past few decades, the commercial lithium-ion batteries (LIBs) have been widely applied in portable electronic devices and electric vehicles (EV)[1-3]. Nowadays, the energy storage devices with high capacity, rapid charging performance and long-period cycling performance are urgently needed to meet the increasing demand of electric vehicles. Unfortunately, the development of LIBs in the field of energy power is greatly limited due to the high lithium metal price and the poor electrochemical performance, such as the poor capacity and rate performance of the commercial’s graphite electrode[4, 5]. In this case, electrode materials with high capacity, excellent rate performance (e.g. faster charge and discharge) and long-period cycling stability at high current density are equally important for the development of LIBs. Transition metal sulfides (e.g. FeS2[6], Sb2S3[7, 8], SnS2[9], ZnS[10], WS2[11], MoS2[12, 13], Ni3S2[14]) with high theoretical specific capacity and abundant sources have been investigated as electrode materials of LIBs and SIBs. Among them, ZnS has been considered to be a potential electrode material by virtue of non-toxicity, high theoretical capacity (LIBs, 962.3 mAh/g), low cost and environmental friendliness[15]. However, just like other transition metal sulfides, ZnS electrode also has significant disadvantages: (1) Serious volume expansion/contraction effect during charge and discharge results in the fall of electrode active materials[16]. (2) The poor electronic conductivity of the bulk ZnS leads to serious electrode polarization[17]. (3) The intercalation/extraction reaction, conversion reaction and alloying reaction of ZnS material during charge and discharge processes lead to serious self-aggregation[18]. At present, many efforts have been made to overcome the above deficiencies. The main strategies can be summarized as follows: (1) Combining ZnS with conductive host materials to construct a composite material such as ZnS/carbon[19], ZnS/graphene[20] and spherical ZnS/carbon[21] with high conductivity to compensate for the poor electrical conductivity of the bulk ZnS and alleviate structure changes and the severe volume effect during electrochemical reactions[22-24]. (2) Rational design of the structure of composite material to ensure the full utilization of the active sites of ZnS as much as possible, and ensure the short insertion/extraction pathway of Li+ during the lithium ion storage process[20]. For example, Daliang Fang et al reported that ZnSQDs@mNC exhibited a reversible capacity of 887 mA h/g after 300 cycles at 840 mA /g and the volume expansion of the ZnS composite electrode material was only 6.5%[25]. Jiabao Li et al in situ-synthesized ZnS nanoparticles on nitrogen-doped porous polyhedral carbon (ZnS/NPC) using metal organic frameworks (ZIF-8) as carbon and nitrogen sources matrix material, and they proved that ZnS/NPC had excellent rate performance (364.6 mAh/g at ble cycling performance in LIBs benefiting from its robust polyhedral structure, excellent electrical conductivity of nitrogen-doped carbon and effective buffer matrix[23]. Although some progress has been made on ZnS materials, there are still some obstacles hindering the development of ZnS and its composites: (1) ZnS
undergoes multi-step electrochemical reactions, which makes ZnS material exhibit more pronounced polarization and lower reversible capacity at high current density. (2) The synthesis of ZnS composite material involves cumbersome experimental procedures and harsh experimental conditions. Therefore, we consider to make holes and defects on the surface of carbon nanotubes by a simple and effective method, allowing metal sulfide to crystallize and grow in holes and defects, so that metal sulfide can be firmly embedded in CNTs to form a robust whole. In this work, multi-walled carbon nanotubes are rationally modified and processed, and then used as the support to obtain a composite of ZnS and CNTs (ZnS-CNTs). Plenty of defects are fabricated on the surfaces of CNTs when the acidified CNTs are etched in air, and the defects of CNTs are proved to provide effective active sites for the growth of ZnS nanoparticles in the hydrothermal reaction process. Eventually, ZnS nanoparticles are firmly decorated on the surfaces of CNTs. On the one hand, CNTs as the basic conductive network provide rapid electron transfer, on the other hand, ZnS nanoparticles uniformly distributed on the surfaces of CNTs provide sufficient electrochemical reaction active sites and rapid ion transport. At the same time, the defects on the surfaces of CNTs provide active sites and adhesion centers for the intermediate products (Zn, LixZn) of ZnS during charge and discharge processes, which not only avoids the agglomeration of ZnS in the recrystallization process, but also increases the utilization efficiency of ZnS. According to the SEM and CV tests results, ZnS nanoparticles tend to be wrapped around the surfaces of CNTs in the earlier stage of the cycling, resulting in obvious structure change of ZnS-CNTs, and the capacitive contribution ratio of ZnS-CNTs electrode is significantly increased during charging-discharging cycling. Finally, the increase of the electrochemical active area effectively avoids the capacity decay caused by the volume effect of ZnS, in turn, the specific capacity of ZnS-CNTs electrode gradually increases. Benefiting from this robust structure, ZnS-CNTs exhibits excellent rate performance and outstanding long-term cycling stability at high current density (reversible capacity of 451.3 mAh/g after 1200 cycles at 5 A/g, 377.8 mAh/g at 8 A/g).
2. Experimental section
2.1. Materials All reagents in this work were analytical grade and used without further purification. The multi-walled carbon nanotubes (L-MWNT-60100, purity > 97%) with about 60-100 nm in diameter and 5-15 µm in length were bought from Shenzhen Nanotech corporation.
2.2. Synthesis of H-O-CNTs Typically, 1g CNTs were added to 40 mL concentrated nitric acid, and the mixture solution was stirred in a constant-temperature oil-bathing for 24 h at 140 °C. Then, the sample was collected by vacuum filtration, washed using ultra-pure water until the filtering solution is neutral, and dried at 70 °C overnight. Finally, H-O-CNTs were obtained by heating the above sample at 568 °C in air for 2 h according to the literature[26].
2.3. Synthesis of ZnS-CNTs 90 mg of H-O-CNTs were slowly added into 60 mL mixed solution of water and ethylene glycol (volume ration 1:1). After ultra-sonication for 1 h, 0.165g (0.75mmol) Zn(CH3COO)2·2H2O, 0.28 g thiourea and 0.109 g hexadecyltrimethyl ammonium bromide were added under magnetic stirring. Then ZnS-CNTs (named as ZnS-CNTs-0.75) was obtained via hydrothermal reaction at 180 °C for 12 h in a 100 mL Teflon-lined stainless autoclave. The sample was finally filtered, washed with ethanol and ultrapure water for several times and freeze-dried overnight. In contrast, ZnS was synthesized by similar process except addition of H-O-CNTs. ZnS-CNTs composites with different ZnS contents are prepared through the same steps and named as ZnS-CNTs-0.5(0.5mmol) and ZnS-CNTs-1.0(1.0 mmol). In order to study the effect of the CNTs etching process in the air on the electrochemical properties of the composite, directly acidified CNTs are also used for the preparation of ZnS-CNTs and the product is named as ZnS-CNTs-Acid.
2.4. Materials characterization Transmission electron microscopy (TEM, Tecnai G2 F20 S-TWIN, Holland) and scanning electron microscopy (SEM, Hitachi S-4800, Japan) were used to investigate the surface morphology. High-resolution transmission electron microscopy (HRTEM) was performed with a Tecnai G2 F20 S-TWIN acceleration voltage operated at 200 kV. X-ray diffractometer (XRD, XRD-6100, Japan) with Cu Ka radiation was applied to study the composition and crystal structure of composite material, NaI was used as crystal scintillation detector. The detection angle (2θ) ranges from 10 to 80 degree. Elemental analysis results were obtained by using X-ray photoelectron spectroscopy (XPS, K-Alpha+, Thermo fisher Scientific) with monochromatic Al Kα (Mono Al Kα) X-ray sources. Raman scattering (invia-reflex) was used to obtain Raman spectroscopy analysis result of the as-prepared material. Thermogravimetric analysis (TGA) result was determined in air with a heating rate of 5 ℃/min from ambient temperature to 790 ℃by SDTQ600. The pore size distribution and specific surface area were obtained by nitrogen adsorption-desorption measurement (JW-BK200C) based on the Barrett-Joyner-Halenda (BJH) and Brunauer-Emmett-Teller model (BET), respectively.
2.5. Electrochemical measurements Electrochemical tests were performed in coin cells (CR2025) with pure Li foil as the counter electrode. The working electrode was prepared by uniform spreading the mixed slurry of active material, carbon black and poly-vinylidene fluoride (PVDF) in N-methylpyrrolidinone solvent with a weight ratio of 7:2:1 on copper foil, and then dried in vacuum at 60 °C for 8 h, with an average mass loading around 1.0 mg/cm2. The LIBs electrolyte was 1 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1 in vol), the separator was glass fiber from whatman (GF/D). All operations were conducted in glove box filled with Ar atmosphere. The discharge and charge were carried out using LANDCT2001A batteries test system (Wuhan, China) with an electrochemical window of 0.01-3 V (vs. Li/Li+) at various current densities. Cyclic
voltammetry tests (CVs) results were obtained on electrochemical workstation (CHI660e, Shanghai) between 0.01 to 3.0 V at various scan rates. Electrochemical impedance (EIS) was performed using the same electrochemical workstation from 0.01 to 100k Hz.
3. Results and discussion
Fig.1 Preparation scheme of ZnS-CNTs composite material
Fig.1 illustrates the synthesis strategy of ZnS-CNTs in detail. Firstly, CNTs were activated by refluxing in concentrated nitric acid (the product was named as O-CNTs). After that, H-O-CNTs were obtained by heating the O-CNTs at 568 °C in air. During these processes, many defects and active sites could be formed in CNTs structure, which is beneficial to the growth of ZnS and the improvement of its electrochemical performance. Finally, ZnS nanoparticles were uniformly grown on H-O-CNTs by a simple hydrothermal process.
Fig.2 (a) XRD pattern of ZnS-CNTs; (b) Raman spectra of CNTs, O-CNTs, H-O-CNTs and ZnS-CNTs; (c) Nitrogen adsorption/desorption curves and pore size distribution of ZnS-CNTs; (d) TGA curve of ZnS-CNTs
The presence of ZnS was verified by the XRD pattern of ZnS-CNTs in Fig.2a, in which the peaks at 2θ = 28.58°, 47.32° and 56.56° are corresponding to the (008), (110) and (118) planes of Wurtzite-8H ZnS (JCPDS No.39-1363) and the peak at 26.22° is corresponding to the lattice plane (002) of graphitic carbon in CNTs (JCPDS No.41-1487)[26, 27]. Fig.2b shows the Raman spectra of CNTs, O-CNTs, H-O-CNTs and ZnS-CNTs. It is obvious that all the samples exhibit typical D
peak (the disorder atoms) and G peak (graphitic atoms) of carbon and the intensity ratio of the two peaks (ID/IG) indicates the disorder degree or graphitization degree of sample[28, 29]. Compared with the ID/IG value of original CNTs (ID/IG = 0.388) , the ID/IG value of O-CNTs (ID/IG = 0.654) is obviously improved, which is mainly because the introduction of oxygen-containing groups on the surface of the pristine CNTs[30]. Meanwhile, the ID/IG value of H-O-CNTs (ID/IG = 0.685) further increases due to the formation of defects in the structure of H-O-CNTs[26]. Compared with the value of H-O-CNTs, the ID/IG value of ZnS-CNTs further increases to 0.742 because of the introduction of ZnS[31, 32]. Brunauer-Emmett-Teller (BET) test was performed to investigate the specific surface area and pore size distribution of ZnS-CNTs, as shown in Fig.2c. The obvious hysteresis loop in the nitrogen adsorption-desorption curve indicates the porous structure of ZnS-CNTs and its specific surface area is calculated to be 175.105 m2/g by the Langmuir analytical method. In addition, the apparent peak appearing in the inserted figure indicates that the diameter of the pores is mainly distributed around 2.747 nm. This porous structure not only provides numerous active sites for the adhesion of ZnS nanoparticles, but also makes the composite have a space to buffer the volume change to some extent in ion insertion/extraction processes during charge and discharge. The content of ZnS in ZnS-CNTs was characterized by TGA. As shown in Fig.2d, the weight loss of ZnS-CNTs undergoes three stages during the heating process in air. The weight loss from 26 ℃ to 200 ℃ is corresponding to the evaporation of adsorbed water[23]. The slope line from 200 ℃ to 560 ℃ is corresponding to the removal of oxygen-containing functional groups on the surfaces of CNTs in composite material[20], and the weight loss from 570℃ to 700℃ is corresponding to the conversion of ZnS into ZnO and the combustion of CNTs. Therefore, the mass ratio of ZnS is roughly calculated to be 37.5% based on the amount of ZnO in the final material composition[33].
Fig.3 Morphology of ZnS-CNTs：(a) SEM image and (b) enlarged SEM image; (c-d) TEM image and corresponding mapping images of C, Zn, S in composite; (e) enlarged TEM image; (f) high resolution TEM image
SEM and TEM were conducted to investigate the morphology and structure changes of CNTs during synthesis process. Compared with the SEM image of CNTs in Fig.S1a, the SEM image in Fig.S1b shows that the ends of O-CNTs are opened, while the ends of raw CNTs are closed (as marked with red circles in Fig.S1a and Fig.S1b), which is consistent with previous literatures[34]. Fig.S1c and Fig.S1d demonstrate the typical SEM and TEM images of H-O-CNTs, a few defects are observed on the surface of H-O-CNTs, which is mainly because a part of carbon on the surface of O-CNTs reacted with oxygen during the heat treatment in air. Fig.3a and 3b show the SEM images of ZnS-CNTs, and it can be observed that lots of bright nanoparticles are distributed on CNTs uniformly. TEM image and corresponding mapping images in Fig.3c and Fig.3d show the distribution of element C, S and Zn in ZnS-CNTs, which illustrate that ZnS nanoparticles are mainly composed of Zn and S, while C is distributed along the CNTs. It can be seen in Fig.3e that ZnS nanoparticles are decorated on the surface of CNT and the HRTEM image of ZnS-CNTs in Fig.3f shows the nanoparticles possess a lattice spacing of 0.19 nm corresponding to the (110) plane of ZnS. All above evidences indicate ZnS nanoparticles are successfully modified on the surfaces of CNTs.
Fig.4 XPS spectra of ZnS-CNTs: (a) survey spectrum; (b) C 1s; (c) Zn 2p and (d) S 2P
The composition of ZnS-CNTs was further investigated by XPS. Fig.4a is the full survey XPS spectrum of ZnS-CNTs and the peaks at 1021.8 eV, 531.1 eV, 284.7 eV and 162.0 eV are corresponding to Zn 2p, O 1s, C 1s and S 2p, respectively. The three peaks located at 284.7 eV, 285.4 eV and 287.3 eV in the XPS spectrum of C 1s (Fig.4b) are corresponding to the bonds energy of C=C, C-O and O-C=O respectively[35]. As shown in Fig.4c, two peaks of the spin orbits of Zn 2p3/2 and Zn 2p1/2 appear at 1021.9 eV and 1045.0 eV, indicating the presence of Zn 2p[36]. Fig.4d is the enlarged S 2p spectrum of ZnS-CNTs, which displays that S 2p peak at 162.0 eV can be divided into two peaks, one is S 2p1/2 spin orbit at 163.2 eV and the other one is S 2p3/2 spin orbit at 161.9 eV[37]. The results of XPS prove the existence of ZnS in ZnS-CNTs once again.
Fig.5 Electrochemical measurements of ZnS-CNTs in LIBs: (a) CV curves from 0.01 to 3 V at a scan rate of 0.1 mV/s; (b) discharge/charge profiles at 0.1A/g; (c) rate performance of ZnS-CNTs; (d) rate performance of ZnS-CNTs after 450 cycles at 4 A/g; (e) cycling performances of ZnS, H-O-CNTs and ZnS-CNTs at 5 A/g
The electrochemical performances of ZnS-CNTs as anode material for LIBs were studied in Fig.5. Fig.5a is the CV curves of ZnS-CNTs at a scan rate of 0.1 mV/s in the range of 0.01-3 V (vs. Li+/Li) and the reduction peaks at about 0.35 V, 0.6 V and 1.65 V in the first cathodic scan are corresponding to the voltage platforms during the first discharge process in Fig.5b. According to previous reports, broad reduction peaks in the potential range of 0.05-0.9 V are ascribed to several processes, such as the transformation reaction of ZnS to metallic Zn and Li2S, the alloying reaction of Zn-Li, and the formation of a solid electrolyte interface (SEI) layer from the decomposition of electrolyte[25]. On the other hand, the oxidation peaks appearing at 0.14 V and 1.35 V in the first anodic scan in Fig.5a are corresponding to the voltage platforms during the first charge process in Fig.5b. These anodic peaks are mainly ascribed to the multistep oxidation or de-lithiation process of Li-Zn alloys (such as LiZn4, LiZn2, Li2Zn5, LiZn and Li2Zn3 etc.)[38] and the typical regeneration of ZnS originating from Zn and Li2S[39, 40]. Besides, a pair of unexpected current peaks appear at 2.38 V and 1.65V in the first scan, which are mainly caused by the ZnO impurity in ZnS-CNTs[41, 42]. In subsequent scans, most of the above peaks including the peaks of ZnO disappeared due to the irreversibility of electrochemical reactions. However, it is obvious that the typical electrochemical peaks of ZnS still exhibited in subsequent cyclic voltammetry scans. Meanwhile, EIS plots of ZnS-CNTs before (A) and after (B) 100 charge-discharge cycles at 0.1A/g are shown in Fig.S2. Typically, the diameter of the semi-circle and the intercept in high frequency range are attributed to the charge transfer resistance (Rct ) and electrolyte resistance (Rs), respectively, and the inclined line in low frequency is correlated with
the Warburg impedance, which is ascribed to the diffusion resistance of Li+ to the surface of electrode[43]. Rct value of ZnS-CNTs electrode decreases from 103.828 Ω to 28.114 Ω, which reveals that the charge-discharge process promotes the compatibility of the active material and electrolyte[31]. Fig.5c shows the rate performance of ZnS-CNTs electrode, the first discharge capacity and charge capacity of ZnS-CNTs electrode are 1042.6 and 563.9 mAh/g, and coulomb efficiency for the first cycle is about 58 %, the capacity loss is mainly caused by the formation of SEI in the first cycle[44]. Reversible capacities of 561.3, 334.6, 290.2, 245.1 and 181.9 mAh/g are obtained at current densities of 0.1, 0.5, 1, 2 and 5 A/g, respectively. When the current density returns to 0.1 A/g, the capacity reaches to 605.4 mA h/g because of the similar activation process of electrode material during cycling. The electrochemical performances of ZnS-CNTs electrodes with different ZnS contents are shown in Fig.S3, it is obvious that the ZnS-CNTs-0.5 and ZnS-CNTs-1.0 electrodes exhibit inferior cycling stability of 162.1 mAh/g and 183.3 mAh/g after 450 cycles at 4A/g and poor recovery capacity at the same current density compared to ZnS-CNTs-0.75, which indicates that the content of ZnS in the composite material affects the rate performance and stability of ZnS-CNTs electrode. If the content of ZnS is too high, the agglomeration of ZnS nanoparticles would like to happen more easily, further causing the decrease of the performance of ZnS-CNTs. On the other hand, the low ZnS content is not good for the capacity and rate performance of the composites. At the same time, in order to study the effect of etching process on the electrochemical properties of the material, the composite material (ZnS-CNTs-Acid) that has not undergone the etching process is prepared by the same method, and corresponding electrochemical performances are shown in Fig.S4. ZnS-CNTs-Acid electrode shows specific capacity of 561.3, 353.8, 299, 252.3 and 181.9 mAh/g at 0.1, 0.5, 1, 2 and 5 A/g respectively. When the current density returns to 0.1 A/g, the capacity can only reach to 410 mA h/g. ZnS-CNTs-Acid electrode delivers poor cycling stability of 42.2 mA h/g after 450 cycles at 4 A/g, which indicates that the application of CNTs etched in air is beneficial to improve the electrochemical lithium ion storage property of the ZnS-CNTs material. In order to further test the activation characteristics of ZnS-CNTs electrode, the rate performance was measured after undergoing 450 cycles at 4 A/g. As shown in Fig.5d, the reversible capacities of 787, 727.4, 654.9, 533.4 and 377.8 mAh/g are obtained at current densities of 0.5, 1, 2, 4 and 8 A/g, respectively. When the current density returns to 0.5A/g, the capacity increases to 913.5 mAh/g. In addition, reversible specific capacity of 493.4 mAh/g is obtained after subsequent 200 cycles at 4 A/g, which indicates that the reversible specific capacity and rate performance of ZnS-CNTs electrode are significantly improved after the activation process. Fig.5e shows the cycling stability of ZnS-CNTs electrode at high current density (5 A/g), the electrode exhibits excellent cycling stability and high reversible specific capacity of 451.3 mA h/g after 1200 cycles. In contrast, pure ZnS electrode shows serious capacity attenuation, while H-O-CNTs electrode exhibits stable cycling performance and low specific capacity. These results indicate that the enhanced performance of ZnS-CNTs electrode benefits from rational structural design and ingenious merge of high specific capacity of ZnS with good cycling stability of CNTs. The excellent electrochemical performance of ZnS-CNTs in this work is one of the best results among the reported research results (See Table 1), especially the rate performance and specific capacities at high current densities. It indicates that this strategy of rational design of CNTs and ZnS is effective for improving the performance of ZnS-based materials.
Table 1 Comparison of the electrochemical performances of ZnS-CNTs with those of the previously reported ZnS-based electrodes in LIBs
Material
Rate performance
ZnS@HPC
370 mAh/g at 1500 mA/g
ZnS NR@HCP
608 mAh/g at 1600 mA/g
ZnS/C
235.2 mAh/g at 800 mA/g
ZnS/graphene
418 mAh/g at 1000 mA/g
ZnS-C nanoparticles
363 mAh/g at 5000 mA/g
ZnS quantum dots@multilayered
~500 mAh/g at 4200 mA/g
carbon ZnS/NPC
364.6 mAh/g at 4000 mA/g
ZnS@NC
380 mAh/g at 2000 mA/g
ZnS quantum dots/graphene
376 mAh/g at 1000 mA/g
Fe-Zn-S@S-doped-C
317 mAh/g at 5000 mA/g
ZnS/PCNFs
438 mAh/g at 2000 mA/g
ZnS@graphite
330 mAh/g at 3000 mA/g
ZnS-CNTs
377.8 mAh/g at 8000 mA/g
cycling performance 408mAh/g at 1000 mA/g after 200 cycles 840 mAh/g at 1600 mA/g after 300 cycles 304.4 mAh/g at 400 mA/g after 300 cycles 570mAh/g at 200 mA/g after 200 cycles 506mAh/g at 500 mA/g after 600 cycles 506mAh/g at 840 mA/g after 300 cycles 856.8mAh/g at 1000 mA/g after 1000 cycles 520mAh/g at 1000 mA/g after 200 cycles 759mAh/g at 100 mA/g after 100 cycles 1321mAh/g at 100 mA/g after 200 cycles 718mAh/g at 200 mA/g after 150 cycles 444mAh/g at 100 mA/g after 300 cycles 451.3mAh/g at 5000 mA/g after 1200 cycles
Reference Ref. [22]
Ref. [45]
Ref. [46]
Ref. [20]
Ref. [16]
Ref. [25]
Ref. [47]
Ref. [48]
Ref. [49]
Ref. [50]
Ref. [51]
Ref. [52]
This work
According to previous reports on transition metal sulfides/carbon-based materials, the reasons for the specific capacity recovery of electrode materials are mainly the following points: (1) Similar activation process in the early stage of cycles could promote Li-diffusion kinetics[53, 54]. (2) After multiple cycles of electrode material, the formation/destruction of the SEI film tends to be stable and eventually active substance is fully utilized, leading to the improvement of cycling performance[23]. (3) The gel-like polymer layer formed at the interface between electrode and electrolyte can contribute excess interfacial storage through pseudo-capacitance behavior[23].
Fig.6 SEM images of ZnS-CNTs after 50, 200, 600 and 1000 cycles at the current density of 5 A/g
However, the reason for the specific capacity recovery of ZnS-C composite material when used as anode material of lithium-ion batteries is not clear, and the mechanisms of the specific capacity recovery may be different for materials with different structure characteristics. In order to illustrate the specific capacity recovery phenomenon of ZnS-CNTs, the morphological and structural transformation of ZnS-CNTs after 50, 200, 600 and 1000 cycles at the current density of 5 A/g were investigated by SEM. As shown in Fig.6, with the charge and discharge going, ZnS nanoparticles (as marked in Fig.6) on the CNTs surface gradually become small and indistinct. Eventually, the ZnS nanoparticles and CNTs merged together. This may result from the fact that ZnS nanoparticles tend to be wrapped around the surfaces of CNTs during the recrystallization process after multiple conversion and intercalation/extraction reactions. This structure that ZnS flattened on the surfaces of CNTs could not only increase the contact area of active materials with electrolyte, but also improve the utilization of ZnS and and eventually increase the specific capacity of the electrode material in subsequent cycles.
Fig.7 CV curves of ZnS-CNTs electrode at scan rates of 0.5, 0.7, 0.9, 1.1, 1.3 and 1.5 mV/s: (a) CV profiles before cycling; (b) CV profiles after 1200 cycles at 5A/g; (c) Log(v) versus Log(i) plot used to calculate the b values at different peaks before cycling; (d) Log(v) versus Log(i) plot used to calculate the b values at different peaks after 1200 cycles at 5A/g; (e) linear fitted curves of v1/2 versus i/v1/2 plots before charge-discharge (A) and after 1200 charge-discharge cycles (B) used to calculate k1 and k2; (f) capacitive contribution ratios before charge-discharge (A) and after 1200 charge-discharge cycles at different scan rates
In order to further investigate the specific capacity recovery of ZnS-CNTs electrode during the charging and discharging from the respect of electrochemical kinetic behavior, cyclic voltammetry tests were conducted at scan rates from 0.5 to 1.5 mV/s before charge-discharge and after 1200 cycles at 5 A/g, and the results are shown in Fig.7a and Fig.7b. It is obviously that the ZnS-CNTs electrode exhibits similar CV curves before and after cycling. Three obvious redox peaks, peak 1, peak 2 and peak 3 labeled in Fig.7a and Fig.7b, are used to further qualitatively estimate the lithium ion storage mechanism by Eq.1: i = avb (1) log(i) = blog(v) + log(a) (2) Where v is the scan rate, i is the peak current, a and b are constants. Typically, the b value approaching to 0.5 indicates an ionic diffusion behavior. While the b value near to 1.0 suggests a
surface-controlled capacitance behavior in lithium-ion storage process[23]. The fitting results are shown in Fig.7c and Fig.7d, the b values of peak 1, peak 2 and peak 3 are 0.799, 0.670 and 0.805 respectively, and the average of b values is 0.758 before cycling, demonstrating that both capacitance behavior and ionic diffusion behavior occur during charge and discharge[55]. However, the b values of peak 1, peak 2 and peak 3 are 0.808, 0.909 and 0.901 respectively, and the average of b values is 0.873 after cycling, indicating that the specific capacity of ZnS-CNTs after cycling is contributed mainly by capacitance behavior. Meanwhile, the quantitative calculation of capacitive contribution ratio during the lithium-ion storage process was also conducted by Eq.3[56-58]. i (V) = k1 v+ k2 v 1/2 (3) where i (V) is the current density at a certain potential, v is the scan rate, k1 and k2 are constants at a certain potential. As shown in Fig.7e, values of k1 and k2 can be calculated through Eq.3, which correspond to the slope and intercept of the fitted lines of v1/2 versus i/v1/2 plots before charge-discharge (A) and after 1200 charge-discharge cycles (B), respectively. The calculation results of capacitive contribution ratio shown in Fig.7f reveal that the capacitive contribution ratio of ZnS-CNTs electrode increases from 47.09% to 58.7% at 0.5 mV/s and 61.92% to 71.38% at 1.5 mV/s after 1200 cycles. In other words, the capacitive contribution ratio of ZnS-CNTs electrode is significantly increased during charging-discharging cycling. The possible reason may be that large ZnS nanoparticles turn to be film-like coating on the surfaces of CNTs, which could increase the reactive area of pseudo-capacitive reaction and eventually enable rapid ion migration during the cycling. This is verified by the SEM images of ZnS-CNTs in Fig.6. In short, the above results indicate that ZnS-CNTs is a promising electrode material with high specific capacity and outstanding cycling stability in lithium-ion storage, the excellent performance could be attributed to the following reasons: (1) Defects from the surfaces of CNTs provide active sites for the adhesion of ZnS nanoparticles during hydrothermal reaction process, which may make the ZnS nanoparticles tightly bind to the CNTs and avoid the obvious volume effect of ZnS due to agglomeration. (2) Active sites and adhesion centers for the intermediate products (Zn, LixZn) of ZnS during Li+ storage process are provided by the defects on the surfaces of CNTs, which not only avoids the agglomeration of ZnS in the recrystallization process, but also increases the utilization efficiency of ZnS. (3) ZnS nanoparticles tend to be wrapped around the surfaces of CNTs during the recrystallization process, which results in larger electrochemical reaction region of ZnS-CNTs than that before cycling. Moreover, capacitive contribution gradually dominates electrode capacity, resulting in more excellent rate performance.
4. Conclusions In conclusion, we synthesized a composite of ZnS-CNTs using etched CNTs as the support and conductive network. The etched CNTs were found to have lots of detects, which could supply many active sites for the growth of ZnS. The robust structure of ZnS-CNTs overcomes the severe polarization of ZnS at high current density during charge and discharge. Benefiting from the higher capacitive contribution ratio than before cycling, the ZnS-CNTs electrode exhibits excellent rate performance and long-period cycling stability at high current densities. A high reversible capacity of 451.3 mAh/g after 1200 cycles at a high current density of 5 A/g, and superior rate
capability (377.8 mAh/g at 8 A/g) were obtained. Furthermore, ZnS-CNTs electrode exhibits the typical specific capacity recovery phenomenon, which is mainly because ZnS nanoparticles gradually extend to the surfaces of CNTs where ZnS nanoparticles are not adhered during repeated electrochemical reactions and recrystallization process. This phenomenon also increases the active region for electrochemical reactions, and makes the ZnS nanoparticles be wrapped on the surfaces of CNTs and fully utilized. In this case, the specific capacity and stability of the composite electrode are greatly improved.
Acknowledgments This work was supported by National Natural Science Foundation of China (Grant No. 51974114, 51672075, 21271069, 51772092) and the Fundamental Research Funds for the Central Universities.
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Highlights
Facile synthesis of ZnS nanoparticles decorated on defective CNTs with excellent performances for lithium-ion batteries anode material Wenlong Zhanga, Zhongyuan Huang*a, Haihui Zhou*a, Songlin Li*b, Chuqing Wanga, Huanxin Lia, Zhanheng Yana, Fei Wanga, Yafei Kuang*a
a
State Key Laboratory for Chemo/Biosensing and Chemometrics, College of
Chemistry and Chemical Engineering, Hunan University, Changsha, Hunan, 410082, China b
State Key Laboratory of Powder Metallurgy, Central South University, Changsha,
Hunan, 410083, China *
Corresponding authors: ([email protected]; [email protected]; [email protected])
[email protected];
ZnS nanoparticles were decorated on defective CNTs through a hydrothermal process. Defective CNTs provide active sites for ZnS and improve electrochemical performance. The morphology change of ZnS results in the capacity recovery of ZnS-CNTs. The capacitive contribution of ZnS-CNTs increased after the long-term cycles.
S0925-8388(19)33879-4
DOI:
https://doi.org/10.1016/j.jallcom.2019.152633
Reference:
JALCOM 152633
To appear in:
Journal of Alloys and Compounds
Received Date: 20 July 2019 Revised Date:
30 September 2019
Accepted Date: 9 October 2019
Please cite this article as: W. Zhang, Z. Huang, H. Zhou, S. Li, C. Wang, H. Li, Z. Yan, F. Wang, Y. Kuang, Facile synthesis of ZnS nanoparticles decorated on defective CNTs with excellent performances for lithium-ion batteries anode material, Journal of Alloys and Compounds (2019), doi: https:// doi.org/10.1016/j.jallcom.2019.152633. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Facile synthesis of ZnS nanoparticles decorated on defective CNTs with excellent performances for lithium-ion batteries anode material Wenlong Zhanga, Zhongyuan Huang*a, Haihui Zhou*a, Songlin Li*b, Chuqing Wanga, Huanxin Lia, Zhanheng Yana, Fei Wanga, Yafei Kuang*a
a
State Key Laboratory for Chemo/Biosensing and Chemometrics, College of
Chemistry and Chemical Engineering, Hunan University, Changsha, Hunan, 410082, China b
State Key Laboratory of Powder Metallurgy, Central South University, Changsha,
Hunan, 410083, China *
Corresponding authors: ([email protected]; [email protected]; [email protected])
[email protected];
Abstract
Zinc sulfide (ZnS) is expected to be a potential replacement of commercial graphite as the anode material for lithium-ion batteries (LIBs) due to its non-toxicity, high theoretical capacity and low cost. Nevertheless, there are still several problems for the application of ZnS during lithium-ion storage process, such as undergoing intercalation/extraction reaction, conversion reaction (ZnS to Zn and LiS2) and alloying reaction (LixZn), which make ZnS exhibit more pronounced polarization and lower reversible capacity at high current density. In order to overcome these problems, in this work we designed a composite (ZnS-CNTs) of etched multi-walled carbon nanotubes (CNTs) and ZnS nanoparticles used as the anode material of LIBs. CNTs were etched in air to obtain rough surfaces with many defects, which could provide many active sites for the growth of ZnS nanoparticles. The ZnS-CNTs composite exhibits high reversible capacity of 451.3 mAh/g after 1200 cycles at a high current density of 5 A/g, and superior rate capability (377.8 mAh/g at 8 A/g). Interestingly, the ZnS-CNTs electrode exhibits the typical specific capacity recovery phenomenon, resulting in an excellent stability during the long-term cycling. SEM images of the ZnS-CNTs electrode materials after different cycles show that the ZnS nanoparticles and CNTs gradually merged together with the increase of cycles, which
not only effectively alleviates the structural damage caused by volume change of ZnS in the charge and discharge cycling, but also greatly increases the specific capacity due to the increase of its electrochemical reaction area.
Keywords: Li ion battery
Anode material
ZnS
Defective CNTs
1. Introduction
Over the past few decades, the commercial lithium-ion batteries (LIBs) have been widely applied in portable electronic devices and electric vehicles (EV)[1-3]. Nowadays, the energy storage devices with high capacity, rapid charging performance and long-period cycling performance are urgently needed to meet the increasing demand of electric vehicles. Unfortunately, the development of LIBs in the field of energy power is greatly limited due to the high lithium metal price and the poor electrochemical performance, such as the poor capacity and rate performance of the commercial’s graphite electrode[4, 5]. In this case, electrode materials with high capacity, excellent rate performance (e.g. faster charge and discharge) and long-period cycling stability at high current density are equally important for the development of LIBs. Transition metal sulfides (e.g. FeS2[6], Sb2S3[7, 8], SnS2[9], ZnS[10], WS2[11], MoS2[12, 13], Ni3S2[14]) with high theoretical specific capacity and abundant sources have been investigated as electrode materials of LIBs and SIBs. Among them, ZnS has been considered to be a potential electrode material by virtue of non-toxicity, high theoretical capacity (LIBs, 962.3 mAh/g), low cost and environmental friendliness[15]. However, just like other transition metal sulfides, ZnS electrode also has significant disadvantages: (1) Serious volume expansion/contraction effect during charge and discharge results in the fall of electrode active materials[16]. (2) The poor electronic conductivity of the bulk ZnS leads to serious electrode polarization[17]. (3) The intercalation/extraction reaction, conversion reaction and alloying reaction of ZnS material during charge and discharge processes lead to serious self-aggregation[18]. At present, many efforts have been made to overcome the above deficiencies. The main strategies can be summarized as follows: (1) Combining ZnS with conductive host materials to construct a composite material such as ZnS/carbon[19], ZnS/graphene[20] and spherical ZnS/carbon[21] with high conductivity to compensate for the poor electrical conductivity of the bulk ZnS and alleviate structure changes and the severe volume effect during electrochemical reactions[22-24]. (2) Rational design of the structure of composite material to ensure the full utilization of the active sites of ZnS as much as possible, and ensure the short insertion/extraction pathway of Li+ during the lithium ion storage process[20]. For example, Daliang Fang et al reported that ZnSQDs@mNC exhibited a reversible capacity of 887 mA h/g after 300 cycles at 840 mA /g and the volume expansion of the ZnS composite electrode material was only 6.5%[25]. Jiabao Li et al in situ-synthesized ZnS nanoparticles on nitrogen-doped porous polyhedral carbon (ZnS/NPC) using metal organic frameworks (ZIF-8) as carbon and nitrogen sources matrix material, and they proved that ZnS/NPC had excellent rate performance (364.6 mAh/g at ble cycling performance in LIBs benefiting from its robust polyhedral structure, excellent electrical conductivity of nitrogen-doped carbon and effective buffer matrix[23]. Although some progress has been made on ZnS materials, there are still some obstacles hindering the development of ZnS and its composites: (1) ZnS
undergoes multi-step electrochemical reactions, which makes ZnS material exhibit more pronounced polarization and lower reversible capacity at high current density. (2) The synthesis of ZnS composite material involves cumbersome experimental procedures and harsh experimental conditions. Therefore, we consider to make holes and defects on the surface of carbon nanotubes by a simple and effective method, allowing metal sulfide to crystallize and grow in holes and defects, so that metal sulfide can be firmly embedded in CNTs to form a robust whole. In this work, multi-walled carbon nanotubes are rationally modified and processed, and then used as the support to obtain a composite of ZnS and CNTs (ZnS-CNTs). Plenty of defects are fabricated on the surfaces of CNTs when the acidified CNTs are etched in air, and the defects of CNTs are proved to provide effective active sites for the growth of ZnS nanoparticles in the hydrothermal reaction process. Eventually, ZnS nanoparticles are firmly decorated on the surfaces of CNTs. On the one hand, CNTs as the basic conductive network provide rapid electron transfer, on the other hand, ZnS nanoparticles uniformly distributed on the surfaces of CNTs provide sufficient electrochemical reaction active sites and rapid ion transport. At the same time, the defects on the surfaces of CNTs provide active sites and adhesion centers for the intermediate products (Zn, LixZn) of ZnS during charge and discharge processes, which not only avoids the agglomeration of ZnS in the recrystallization process, but also increases the utilization efficiency of ZnS. According to the SEM and CV tests results, ZnS nanoparticles tend to be wrapped around the surfaces of CNTs in the earlier stage of the cycling, resulting in obvious structure change of ZnS-CNTs, and the capacitive contribution ratio of ZnS-CNTs electrode is significantly increased during charging-discharging cycling. Finally, the increase of the electrochemical active area effectively avoids the capacity decay caused by the volume effect of ZnS, in turn, the specific capacity of ZnS-CNTs electrode gradually increases. Benefiting from this robust structure, ZnS-CNTs exhibits excellent rate performance and outstanding long-term cycling stability at high current density (reversible capacity of 451.3 mAh/g after 1200 cycles at 5 A/g, 377.8 mAh/g at 8 A/g).
2. Experimental section
2.1. Materials All reagents in this work were analytical grade and used without further purification. The multi-walled carbon nanotubes (L-MWNT-60100, purity > 97%) with about 60-100 nm in diameter and 5-15 µm in length were bought from Shenzhen Nanotech corporation.
2.2. Synthesis of H-O-CNTs Typically, 1g CNTs were added to 40 mL concentrated nitric acid, and the mixture solution was stirred in a constant-temperature oil-bathing for 24 h at 140 °C. Then, the sample was collected by vacuum filtration, washed using ultra-pure water until the filtering solution is neutral, and dried at 70 °C overnight. Finally, H-O-CNTs were obtained by heating the above sample at 568 °C in air for 2 h according to the literature[26].
2.3. Synthesis of ZnS-CNTs 90 mg of H-O-CNTs were slowly added into 60 mL mixed solution of water and ethylene glycol (volume ration 1:1). After ultra-sonication for 1 h, 0.165g (0.75mmol) Zn(CH3COO)2·2H2O, 0.28 g thiourea and 0.109 g hexadecyltrimethyl ammonium bromide were added under magnetic stirring. Then ZnS-CNTs (named as ZnS-CNTs-0.75) was obtained via hydrothermal reaction at 180 °C for 12 h in a 100 mL Teflon-lined stainless autoclave. The sample was finally filtered, washed with ethanol and ultrapure water for several times and freeze-dried overnight. In contrast, ZnS was synthesized by similar process except addition of H-O-CNTs. ZnS-CNTs composites with different ZnS contents are prepared through the same steps and named as ZnS-CNTs-0.5(0.5mmol) and ZnS-CNTs-1.0(1.0 mmol). In order to study the effect of the CNTs etching process in the air on the electrochemical properties of the composite, directly acidified CNTs are also used for the preparation of ZnS-CNTs and the product is named as ZnS-CNTs-Acid.
2.4. Materials characterization Transmission electron microscopy (TEM, Tecnai G2 F20 S-TWIN, Holland) and scanning electron microscopy (SEM, Hitachi S-4800, Japan) were used to investigate the surface morphology. High-resolution transmission electron microscopy (HRTEM) was performed with a Tecnai G2 F20 S-TWIN acceleration voltage operated at 200 kV. X-ray diffractometer (XRD, XRD-6100, Japan) with Cu Ka radiation was applied to study the composition and crystal structure of composite material, NaI was used as crystal scintillation detector. The detection angle (2θ) ranges from 10 to 80 degree. Elemental analysis results were obtained by using X-ray photoelectron spectroscopy (XPS, K-Alpha+, Thermo fisher Scientific) with monochromatic Al Kα (Mono Al Kα) X-ray sources. Raman scattering (invia-reflex) was used to obtain Raman spectroscopy analysis result of the as-prepared material. Thermogravimetric analysis (TGA) result was determined in air with a heating rate of 5 ℃/min from ambient temperature to 790 ℃by SDTQ600. The pore size distribution and specific surface area were obtained by nitrogen adsorption-desorption measurement (JW-BK200C) based on the Barrett-Joyner-Halenda (BJH) and Brunauer-Emmett-Teller model (BET), respectively.
2.5. Electrochemical measurements Electrochemical tests were performed in coin cells (CR2025) with pure Li foil as the counter electrode. The working electrode was prepared by uniform spreading the mixed slurry of active material, carbon black and poly-vinylidene fluoride (PVDF) in N-methylpyrrolidinone solvent with a weight ratio of 7:2:1 on copper foil, and then dried in vacuum at 60 °C for 8 h, with an average mass loading around 1.0 mg/cm2. The LIBs electrolyte was 1 M LiPF6 in ethylene carbonate (EC)/dimethyl carbonate (DMC) (1:1 in vol), the separator was glass fiber from whatman (GF/D). All operations were conducted in glove box filled with Ar atmosphere. The discharge and charge were carried out using LANDCT2001A batteries test system (Wuhan, China) with an electrochemical window of 0.01-3 V (vs. Li/Li+) at various current densities. Cyclic
voltammetry tests (CVs) results were obtained on electrochemical workstation (CHI660e, Shanghai) between 0.01 to 3.0 V at various scan rates. Electrochemical impedance (EIS) was performed using the same electrochemical workstation from 0.01 to 100k Hz.
3. Results and discussion
Fig.1 Preparation scheme of ZnS-CNTs composite material
Fig.1 illustrates the synthesis strategy of ZnS-CNTs in detail. Firstly, CNTs were activated by refluxing in concentrated nitric acid (the product was named as O-CNTs). After that, H-O-CNTs were obtained by heating the O-CNTs at 568 °C in air. During these processes, many defects and active sites could be formed in CNTs structure, which is beneficial to the growth of ZnS and the improvement of its electrochemical performance. Finally, ZnS nanoparticles were uniformly grown on H-O-CNTs by a simple hydrothermal process.
Fig.2 (a) XRD pattern of ZnS-CNTs; (b) Raman spectra of CNTs, O-CNTs, H-O-CNTs and ZnS-CNTs; (c) Nitrogen adsorption/desorption curves and pore size distribution of ZnS-CNTs; (d) TGA curve of ZnS-CNTs
The presence of ZnS was verified by the XRD pattern of ZnS-CNTs in Fig.2a, in which the peaks at 2θ = 28.58°, 47.32° and 56.56° are corresponding to the (008), (110) and (118) planes of Wurtzite-8H ZnS (JCPDS No.39-1363) and the peak at 26.22° is corresponding to the lattice plane (002) of graphitic carbon in CNTs (JCPDS No.41-1487)[26, 27]. Fig.2b shows the Raman spectra of CNTs, O-CNTs, H-O-CNTs and ZnS-CNTs. It is obvious that all the samples exhibit typical D
peak (the disorder atoms) and G peak (graphitic atoms) of carbon and the intensity ratio of the two peaks (ID/IG) indicates the disorder degree or graphitization degree of sample[28, 29]. Compared with the ID/IG value of original CNTs (ID/IG = 0.388) , the ID/IG value of O-CNTs (ID/IG = 0.654) is obviously improved, which is mainly because the introduction of oxygen-containing groups on the surface of the pristine CNTs[30]. Meanwhile, the ID/IG value of H-O-CNTs (ID/IG = 0.685) further increases due to the formation of defects in the structure of H-O-CNTs[26]. Compared with the value of H-O-CNTs, the ID/IG value of ZnS-CNTs further increases to 0.742 because of the introduction of ZnS[31, 32]. Brunauer-Emmett-Teller (BET) test was performed to investigate the specific surface area and pore size distribution of ZnS-CNTs, as shown in Fig.2c. The obvious hysteresis loop in the nitrogen adsorption-desorption curve indicates the porous structure of ZnS-CNTs and its specific surface area is calculated to be 175.105 m2/g by the Langmuir analytical method. In addition, the apparent peak appearing in the inserted figure indicates that the diameter of the pores is mainly distributed around 2.747 nm. This porous structure not only provides numerous active sites for the adhesion of ZnS nanoparticles, but also makes the composite have a space to buffer the volume change to some extent in ion insertion/extraction processes during charge and discharge. The content of ZnS in ZnS-CNTs was characterized by TGA. As shown in Fig.2d, the weight loss of ZnS-CNTs undergoes three stages during the heating process in air. The weight loss from 26 ℃ to 200 ℃ is corresponding to the evaporation of adsorbed water[23]. The slope line from 200 ℃ to 560 ℃ is corresponding to the removal of oxygen-containing functional groups on the surfaces of CNTs in composite material[20], and the weight loss from 570℃ to 700℃ is corresponding to the conversion of ZnS into ZnO and the combustion of CNTs. Therefore, the mass ratio of ZnS is roughly calculated to be 37.5% based on the amount of ZnO in the final material composition[33].
Fig.3 Morphology of ZnS-CNTs：(a) SEM image and (b) enlarged SEM image; (c-d) TEM image and corresponding mapping images of C, Zn, S in composite; (e) enlarged TEM image; (f) high resolution TEM image
SEM and TEM were conducted to investigate the morphology and structure changes of CNTs during synthesis process. Compared with the SEM image of CNTs in Fig.S1a, the SEM image in Fig.S1b shows that the ends of O-CNTs are opened, while the ends of raw CNTs are closed (as marked with red circles in Fig.S1a and Fig.S1b), which is consistent with previous literatures[34]. Fig.S1c and Fig.S1d demonstrate the typical SEM and TEM images of H-O-CNTs, a few defects are observed on the surface of H-O-CNTs, which is mainly because a part of carbon on the surface of O-CNTs reacted with oxygen during the heat treatment in air. Fig.3a and 3b show the SEM images of ZnS-CNTs, and it can be observed that lots of bright nanoparticles are distributed on CNTs uniformly. TEM image and corresponding mapping images in Fig.3c and Fig.3d show the distribution of element C, S and Zn in ZnS-CNTs, which illustrate that ZnS nanoparticles are mainly composed of Zn and S, while C is distributed along the CNTs. It can be seen in Fig.3e that ZnS nanoparticles are decorated on the surface of CNT and the HRTEM image of ZnS-CNTs in Fig.3f shows the nanoparticles possess a lattice spacing of 0.19 nm corresponding to the (110) plane of ZnS. All above evidences indicate ZnS nanoparticles are successfully modified on the surfaces of CNTs.
Fig.4 XPS spectra of ZnS-CNTs: (a) survey spectrum; (b) C 1s; (c) Zn 2p and (d) S 2P
The composition of ZnS-CNTs was further investigated by XPS. Fig.4a is the full survey XPS spectrum of ZnS-CNTs and the peaks at 1021.8 eV, 531.1 eV, 284.7 eV and 162.0 eV are corresponding to Zn 2p, O 1s, C 1s and S 2p, respectively. The three peaks located at 284.7 eV, 285.4 eV and 287.3 eV in the XPS spectrum of C 1s (Fig.4b) are corresponding to the bonds energy of C=C, C-O and O-C=O respectively[35]. As shown in Fig.4c, two peaks of the spin orbits of Zn 2p3/2 and Zn 2p1/2 appear at 1021.9 eV and 1045.0 eV, indicating the presence of Zn 2p[36]. Fig.4d is the enlarged S 2p spectrum of ZnS-CNTs, which displays that S 2p peak at 162.0 eV can be divided into two peaks, one is S 2p1/2 spin orbit at 163.2 eV and the other one is S 2p3/2 spin orbit at 161.9 eV[37]. The results of XPS prove the existence of ZnS in ZnS-CNTs once again.
Fig.5 Electrochemical measurements of ZnS-CNTs in LIBs: (a) CV curves from 0.01 to 3 V at a scan rate of 0.1 mV/s; (b) discharge/charge profiles at 0.1A/g; (c) rate performance of ZnS-CNTs; (d) rate performance of ZnS-CNTs after 450 cycles at 4 A/g; (e) cycling performances of ZnS, H-O-CNTs and ZnS-CNTs at 5 A/g
The electrochemical performances of ZnS-CNTs as anode material for LIBs were studied in Fig.5. Fig.5a is the CV curves of ZnS-CNTs at a scan rate of 0.1 mV/s in the range of 0.01-3 V (vs. Li+/Li) and the reduction peaks at about 0.35 V, 0.6 V and 1.65 V in the first cathodic scan are corresponding to the voltage platforms during the first discharge process in Fig.5b. According to previous reports, broad reduction peaks in the potential range of 0.05-0.9 V are ascribed to several processes, such as the transformation reaction of ZnS to metallic Zn and Li2S, the alloying reaction of Zn-Li, and the formation of a solid electrolyte interface (SEI) layer from the decomposition of electrolyte[25]. On the other hand, the oxidation peaks appearing at 0.14 V and 1.35 V in the first anodic scan in Fig.5a are corresponding to the voltage platforms during the first charge process in Fig.5b. These anodic peaks are mainly ascribed to the multistep oxidation or de-lithiation process of Li-Zn alloys (such as LiZn4, LiZn2, Li2Zn5, LiZn and Li2Zn3 etc.)[38] and the typical regeneration of ZnS originating from Zn and Li2S[39, 40]. Besides, a pair of unexpected current peaks appear at 2.38 V and 1.65V in the first scan, which are mainly caused by the ZnO impurity in ZnS-CNTs[41, 42]. In subsequent scans, most of the above peaks including the peaks of ZnO disappeared due to the irreversibility of electrochemical reactions. However, it is obvious that the typical electrochemical peaks of ZnS still exhibited in subsequent cyclic voltammetry scans. Meanwhile, EIS plots of ZnS-CNTs before (A) and after (B) 100 charge-discharge cycles at 0.1A/g are shown in Fig.S2. Typically, the diameter of the semi-circle and the intercept in high frequency range are attributed to the charge transfer resistance (Rct ) and electrolyte resistance (Rs), respectively, and the inclined line in low frequency is correlated with
the Warburg impedance, which is ascribed to the diffusion resistance of Li+ to the surface of electrode[43]. Rct value of ZnS-CNTs electrode decreases from 103.828 Ω to 28.114 Ω, which reveals that the charge-discharge process promotes the compatibility of the active material and electrolyte[31]. Fig.5c shows the rate performance of ZnS-CNTs electrode, the first discharge capacity and charge capacity of ZnS-CNTs electrode are 1042.6 and 563.9 mAh/g, and coulomb efficiency for the first cycle is about 58 %, the capacity loss is mainly caused by the formation of SEI in the first cycle[44]. Reversible capacities of 561.3, 334.6, 290.2, 245.1 and 181.9 mAh/g are obtained at current densities of 0.1, 0.5, 1, 2 and 5 A/g, respectively. When the current density returns to 0.1 A/g, the capacity reaches to 605.4 mA h/g because of the similar activation process of electrode material during cycling. The electrochemical performances of ZnS-CNTs electrodes with different ZnS contents are shown in Fig.S3, it is obvious that the ZnS-CNTs-0.5 and ZnS-CNTs-1.0 electrodes exhibit inferior cycling stability of 162.1 mAh/g and 183.3 mAh/g after 450 cycles at 4A/g and poor recovery capacity at the same current density compared to ZnS-CNTs-0.75, which indicates that the content of ZnS in the composite material affects the rate performance and stability of ZnS-CNTs electrode. If the content of ZnS is too high, the agglomeration of ZnS nanoparticles would like to happen more easily, further causing the decrease of the performance of ZnS-CNTs. On the other hand, the low ZnS content is not good for the capacity and rate performance of the composites. At the same time, in order to study the effect of etching process on the electrochemical properties of the material, the composite material (ZnS-CNTs-Acid) that has not undergone the etching process is prepared by the same method, and corresponding electrochemical performances are shown in Fig.S4. ZnS-CNTs-Acid electrode shows specific capacity of 561.3, 353.8, 299, 252.3 and 181.9 mAh/g at 0.1, 0.5, 1, 2 and 5 A/g respectively. When the current density returns to 0.1 A/g, the capacity can only reach to 410 mA h/g. ZnS-CNTs-Acid electrode delivers poor cycling stability of 42.2 mA h/g after 450 cycles at 4 A/g, which indicates that the application of CNTs etched in air is beneficial to improve the electrochemical lithium ion storage property of the ZnS-CNTs material. In order to further test the activation characteristics of ZnS-CNTs electrode, the rate performance was measured after undergoing 450 cycles at 4 A/g. As shown in Fig.5d, the reversible capacities of 787, 727.4, 654.9, 533.4 and 377.8 mAh/g are obtained at current densities of 0.5, 1, 2, 4 and 8 A/g, respectively. When the current density returns to 0.5A/g, the capacity increases to 913.5 mAh/g. In addition, reversible specific capacity of 493.4 mAh/g is obtained after subsequent 200 cycles at 4 A/g, which indicates that the reversible specific capacity and rate performance of ZnS-CNTs electrode are significantly improved after the activation process. Fig.5e shows the cycling stability of ZnS-CNTs electrode at high current density (5 A/g), the electrode exhibits excellent cycling stability and high reversible specific capacity of 451.3 mA h/g after 1200 cycles. In contrast, pure ZnS electrode shows serious capacity attenuation, while H-O-CNTs electrode exhibits stable cycling performance and low specific capacity. These results indicate that the enhanced performance of ZnS-CNTs electrode benefits from rational structural design and ingenious merge of high specific capacity of ZnS with good cycling stability of CNTs. The excellent electrochemical performance of ZnS-CNTs in this work is one of the best results among the reported research results (See Table 1), especially the rate performance and specific capacities at high current densities. It indicates that this strategy of rational design of CNTs and ZnS is effective for improving the performance of ZnS-based materials.
Table 1 Comparison of the electrochemical performances of ZnS-CNTs with those of the previously reported ZnS-based electrodes in LIBs
Material
Rate performance
ZnS@HPC
370 mAh/g at 1500 mA/g
ZnS NR@HCP
608 mAh/g at 1600 mA/g
ZnS/C
235.2 mAh/g at 800 mA/g
ZnS/graphene
418 mAh/g at 1000 mA/g
ZnS-C nanoparticles
363 mAh/g at 5000 mA/g
ZnS quantum dots@multilayered
~500 mAh/g at 4200 mA/g
carbon ZnS/NPC
364.6 mAh/g at 4000 mA/g
ZnS@NC
380 mAh/g at 2000 mA/g
ZnS quantum dots/graphene
376 mAh/g at 1000 mA/g
Fe-Zn-S@S-doped-C
317 mAh/g at 5000 mA/g
ZnS/PCNFs
438 mAh/g at 2000 mA/g
ZnS@graphite
330 mAh/g at 3000 mA/g
ZnS-CNTs
377.8 mAh/g at 8000 mA/g
cycling performance 408mAh/g at 1000 mA/g after 200 cycles 840 mAh/g at 1600 mA/g after 300 cycles 304.4 mAh/g at 400 mA/g after 300 cycles 570mAh/g at 200 mA/g after 200 cycles 506mAh/g at 500 mA/g after 600 cycles 506mAh/g at 840 mA/g after 300 cycles 856.8mAh/g at 1000 mA/g after 1000 cycles 520mAh/g at 1000 mA/g after 200 cycles 759mAh/g at 100 mA/g after 100 cycles 1321mAh/g at 100 mA/g after 200 cycles 718mAh/g at 200 mA/g after 150 cycles 444mAh/g at 100 mA/g after 300 cycles 451.3mAh/g at 5000 mA/g after 1200 cycles
Reference Ref. [22]
Ref. [45]
Ref. [46]
Ref. [20]
Ref. [16]
Ref. [25]
Ref. [47]
Ref. [48]
Ref. [49]
Ref. [50]
Ref. [51]
Ref. [52]
This work
According to previous reports on transition metal sulfides/carbon-based materials, the reasons for the specific capacity recovery of electrode materials are mainly the following points: (1) Similar activation process in the early stage of cycles could promote Li-diffusion kinetics[53, 54]. (2) After multiple cycles of electrode material, the formation/destruction of the SEI film tends to be stable and eventually active substance is fully utilized, leading to the improvement of cycling performance[23]. (3) The gel-like polymer layer formed at the interface between electrode and electrolyte can contribute excess interfacial storage through pseudo-capacitance behavior[23].
Fig.6 SEM images of ZnS-CNTs after 50, 200, 600 and 1000 cycles at the current density of 5 A/g
However, the reason for the specific capacity recovery of ZnS-C composite material when used as anode material of lithium-ion batteries is not clear, and the mechanisms of the specific capacity recovery may be different for materials with different structure characteristics. In order to illustrate the specific capacity recovery phenomenon of ZnS-CNTs, the morphological and structural transformation of ZnS-CNTs after 50, 200, 600 and 1000 cycles at the current density of 5 A/g were investigated by SEM. As shown in Fig.6, with the charge and discharge going, ZnS nanoparticles (as marked in Fig.6) on the CNTs surface gradually become small and indistinct. Eventually, the ZnS nanoparticles and CNTs merged together. This may result from the fact that ZnS nanoparticles tend to be wrapped around the surfaces of CNTs during the recrystallization process after multiple conversion and intercalation/extraction reactions. This structure that ZnS flattened on the surfaces of CNTs could not only increase the contact area of active materials with electrolyte, but also improve the utilization of ZnS and and eventually increase the specific capacity of the electrode material in subsequent cycles.
Fig.7 CV curves of ZnS-CNTs electrode at scan rates of 0.5, 0.7, 0.9, 1.1, 1.3 and 1.5 mV/s: (a) CV profiles before cycling; (b) CV profiles after 1200 cycles at 5A/g; (c) Log(v) versus Log(i) plot used to calculate the b values at different peaks before cycling; (d) Log(v) versus Log(i) plot used to calculate the b values at different peaks after 1200 cycles at 5A/g; (e) linear fitted curves of v1/2 versus i/v1/2 plots before charge-discharge (A) and after 1200 charge-discharge cycles (B) used to calculate k1 and k2; (f) capacitive contribution ratios before charge-discharge (A) and after 1200 charge-discharge cycles at different scan rates
In order to further investigate the specific capacity recovery of ZnS-CNTs electrode during the charging and discharging from the respect of electrochemical kinetic behavior, cyclic voltammetry tests were conducted at scan rates from 0.5 to 1.5 mV/s before charge-discharge and after 1200 cycles at 5 A/g, and the results are shown in Fig.7a and Fig.7b. It is obviously that the ZnS-CNTs electrode exhibits similar CV curves before and after cycling. Three obvious redox peaks, peak 1, peak 2 and peak 3 labeled in Fig.7a and Fig.7b, are used to further qualitatively estimate the lithium ion storage mechanism by Eq.1: i = avb (1) log(i) = blog(v) + log(a) (2) Where v is the scan rate, i is the peak current, a and b are constants. Typically, the b value approaching to 0.5 indicates an ionic diffusion behavior. While the b value near to 1.0 suggests a
surface-controlled capacitance behavior in lithium-ion storage process[23]. The fitting results are shown in Fig.7c and Fig.7d, the b values of peak 1, peak 2 and peak 3 are 0.799, 0.670 and 0.805 respectively, and the average of b values is 0.758 before cycling, demonstrating that both capacitance behavior and ionic diffusion behavior occur during charge and discharge[55]. However, the b values of peak 1, peak 2 and peak 3 are 0.808, 0.909 and 0.901 respectively, and the average of b values is 0.873 after cycling, indicating that the specific capacity of ZnS-CNTs after cycling is contributed mainly by capacitance behavior. Meanwhile, the quantitative calculation of capacitive contribution ratio during the lithium-ion storage process was also conducted by Eq.3[56-58]. i (V) = k1 v+ k2 v 1/2 (3) where i (V) is the current density at a certain potential, v is the scan rate, k1 and k2 are constants at a certain potential. As shown in Fig.7e, values of k1 and k2 can be calculated through Eq.3, which correspond to the slope and intercept of the fitted lines of v1/2 versus i/v1/2 plots before charge-discharge (A) and after 1200 charge-discharge cycles (B), respectively. The calculation results of capacitive contribution ratio shown in Fig.7f reveal that the capacitive contribution ratio of ZnS-CNTs electrode increases from 47.09% to 58.7% at 0.5 mV/s and 61.92% to 71.38% at 1.5 mV/s after 1200 cycles. In other words, the capacitive contribution ratio of ZnS-CNTs electrode is significantly increased during charging-discharging cycling. The possible reason may be that large ZnS nanoparticles turn to be film-like coating on the surfaces of CNTs, which could increase the reactive area of pseudo-capacitive reaction and eventually enable rapid ion migration during the cycling. This is verified by the SEM images of ZnS-CNTs in Fig.6. In short, the above results indicate that ZnS-CNTs is a promising electrode material with high specific capacity and outstanding cycling stability in lithium-ion storage, the excellent performance could be attributed to the following reasons: (1) Defects from the surfaces of CNTs provide active sites for the adhesion of ZnS nanoparticles during hydrothermal reaction process, which may make the ZnS nanoparticles tightly bind to the CNTs and avoid the obvious volume effect of ZnS due to agglomeration. (2) Active sites and adhesion centers for the intermediate products (Zn, LixZn) of ZnS during Li+ storage process are provided by the defects on the surfaces of CNTs, which not only avoids the agglomeration of ZnS in the recrystallization process, but also increases the utilization efficiency of ZnS. (3) ZnS nanoparticles tend to be wrapped around the surfaces of CNTs during the recrystallization process, which results in larger electrochemical reaction region of ZnS-CNTs than that before cycling. Moreover, capacitive contribution gradually dominates electrode capacity, resulting in more excellent rate performance.
4. Conclusions In conclusion, we synthesized a composite of ZnS-CNTs using etched CNTs as the support and conductive network. The etched CNTs were found to have lots of detects, which could supply many active sites for the growth of ZnS. The robust structure of ZnS-CNTs overcomes the severe polarization of ZnS at high current density during charge and discharge. Benefiting from the higher capacitive contribution ratio than before cycling, the ZnS-CNTs electrode exhibits excellent rate performance and long-period cycling stability at high current densities. A high reversible capacity of 451.3 mAh/g after 1200 cycles at a high current density of 5 A/g, and superior rate
capability (377.8 mAh/g at 8 A/g) were obtained. Furthermore, ZnS-CNTs electrode exhibits the typical specific capacity recovery phenomenon, which is mainly because ZnS nanoparticles gradually extend to the surfaces of CNTs where ZnS nanoparticles are not adhered during repeated electrochemical reactions and recrystallization process. This phenomenon also increases the active region for electrochemical reactions, and makes the ZnS nanoparticles be wrapped on the surfaces of CNTs and fully utilized. In this case, the specific capacity and stability of the composite electrode are greatly improved.
Acknowledgments This work was supported by National Natural Science Foundation of China (Grant No. 51974114, 51672075, 21271069, 51772092) and the Fundamental Research Funds for the Central Universities.
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Highlights
Facile synthesis of ZnS nanoparticles decorated on defective CNTs with excellent performances for lithium-ion batteries anode material Wenlong Zhanga, Zhongyuan Huang*a, Haihui Zhou*a, Songlin Li*b, Chuqing Wanga, Huanxin Lia, Zhanheng Yana, Fei Wanga, Yafei Kuang*a
a
State Key Laboratory for Chemo/Biosensing and Chemometrics, College of
Chemistry and Chemical Engineering, Hunan University, Changsha, Hunan, 410082, China b
State Key Laboratory of Powder Metallurgy, Central South University, Changsha,
Hunan, 410083, China *
Corresponding authors: ([email protected]; [email protected]; [email protected])
[email protected];
ZnS nanoparticles were decorated on defective CNTs through a hydrothermal process. Defective CNTs provide active sites for ZnS and improve electrochemical performance. The morphology change of ZnS results in the capacity recovery of ZnS-CNTs. The capacitive contribution of ZnS-CNTs increased after the long-term cycles.